In optical communication systems that employ coherent optical receivers, the modulated optical signal received at the coherent receiver is mixed with a narrow-line-width local oscillator (LO) signal, and the combined signal is made incident on one or more photodetectors. The frequency spectrum of the electrical current appearing at the photodetector output(s) is substantially proportional to the convolution of the received optical signal spectrum and the local oscillator (LO) spectrum, and contains a signal component lying at an intermediate frequency that contains data modulated onto the received signal. Consequently, this “data component” can be isolated and detected by electronically filtering and processing the photodetector output current.
FIG. 1 schematically illustrates an optical communications system which incorporates a representative coherent optical receiver known in the art, and, for example, from U.S. Pat. No. 7,606,498.
In the optical communications system of FIG. 1, the transmitter 2 comprises a Tx local oscillator laser 4, which generates a narrow-band optical carrier light having a center frequency of f0. This optical carrier light is modulated by an optical modulator 6 in accordance with a drive signal S(t), which includes encoded data signal x(t) having a bandwidth of fB (where f0>>fB). The baseband data signal x(t) can be generally represented as x(t)=xI(t)+jxQ(t), where xI(t) and xQ(t) respectively represent the In-Phase and Quadrature (or, equivalently, the Real (Re) and Imaginary (Im)) components of the baseband data signal x(t). As may be appreciated, the baseband data signal x(t) may be either real or complex. Where x(t) is a complex signal (such as, for example, a M-ary phase shift keying signal) the value of each encoded symbol is represented by either one or both of the amplitude and phase. In such cases, the optical modulator 6 is configured to modulate both amplitude and phase of the narrow-band optical carrier light in accordance with the data signal x(t) to generate the modulated optical signal y(t). Where x(t) is a real signal (such as, for example, a multi-level Amplitude Shift Keying (ASK) signal), the value of an encoded symbol is represented by the analog amplitude level, thus xQ(t)=0. In such cases, the optical modulator 6 is configured to modulate the amplitude of the narrow-band optical carrier light in accordance with the data signal x(t) to generate the modulated optical signal y(t).
The modulated optical signal y(t) is transmitted through an optical fibre link 8 to the coherent optical receiver 10. Typically, the optical fibre link 8 will include multiple optical fibre spans cascaded in series with various optical equipment including, for example, optical amplifiers, Optical Add-Drop Multiplexers (OADMs) etc.
In the coherent optical receiver 10 of FIG. 1, the inbound optical signal y(t) received through the optical link 8 is split into orthogonal received polarizations X, Y by a Polarization Beam Splitter 12. The received X and Y polarizations are then supplied to a conventional 90° optical hybrid 14, and mixed with Local Oscillator (LO) light having a frequency of f1 generated by an receiver LO laser 16 and the composite lights emerging from the optical hybrid 14 supplied to a respective photodetector 18, which generates a corresponding analog detector signal. Typically, each photodetector 18 is provided as a balanced pair of P-Intrinsic-N (PIN) diodes, and the analog current of the corresponding detector signal is proportional to the optical power of the incident composite light. Each of the analog detector signals output by the photodetectors 18 is sampled by a respective Analog-to-Digital (A/D) converter 20, to yield multi-bit digital I and Q raw sample streams for each of the received X and Y polarizations. In order to avoid aliasing errors, Nyquist sampling is typically used, in which the sample rate fS of the A/D converter 20 is about twice the band-width fB of the received optical signal. From the A/D converter 20 block, the I and Q raw sample streams of each received polarization are supplied to digital signal processor (DSP) 22 for data and carrier recovery using methods known in the art.
FIG. 2a illustrates, in greater detail, the optical hybrid 14 for the X polarization. This arrangement is duplicated for the Y-polarization. As may be seen in FIG. 2a, for each polarization, the optical hybrid 14 implements a homodyne optical downconverter comprising a set of four balanced optical mixers 24, which are arranged such that the received polarization light is mixed with the Rx LO light and a 90° phase-shifted version of the Rx LO light, to generate the composite lights that are made incident of the photodetectors 18. By controlling the Rx LO 16 to output the Rx LO light frequency f1=f0, and further controlling the polarization angles of the received X polarization light and the Rx LO lights propagating through the optical mixers 24, the composite light output from the hybrid 14 will contain base-band beat products (centered at around 0 Hz) corresponding to the original drive signal x(t). In some cases, optical polarization controllers (not shown) are used to ensure the required alignment between the polarization angles of the X polarization light and the Rx LO lights. With this arrangement, driving the A/D converters 20 to sample the analog detector signals output by the photodetectors 18 at a sample rate fS≈2fB yields In-Phase and Quadrature raw sample streams IX, QX from which the encoded data signal x(t) can be recovered using known digital signal processing techniques.
A limitation of the arrangement of FIG. 2a, is that the need for multiple optical mixers 24 along with a 90° phase shifter and one or more optical polarization controllers makes the 90° optical hybrid 14 very expensive. FIG. 2b illustrates an alternative arrangement which utilizes fewer high-cost optical components.
The receiver of FIG. 2b implements an optical heterodyne downconverter, which utilizes a single optical mixer 24 (and, possibly, a polarization controller to ensure alignment) which mixes the received X polarization light with the Rx LO light, to generate a composite light that is made incident on a single photodetector 18. By controlling the Rx LO 6 to output the Rx LO light with a frequency f1 at a desired offset Δf from the Tx optical carrier frequency f0, the composite light output from the mixer 24 will contain Intermediate Frequency (IF) beat products (centered at around Δf) corresponding to the original drive signal x(t). The corresponding IF signal components in the analog detector signal output by the photodetector 18 can then be down-converted to In-phase and Quadrature baseband signals using an electronic oscillator 26 to generate an oscillator signal having a frequency f2 and analog mixers 28 for mixing the analog detector signal with the oscillator signal and a 90° phase shifted version of the oscillator signal. As in the receiver of FIG. 2a, driving the A/D converters to sample the mixer output signals at a sample rate fS≈2fB yields In-Phase and Quadrature raw sample streams IX, QX from which the encoded data signal x(t) can be recovered using known digital signal processing techniques.
A limitation of the arrangement of FIG. 2b is that generation of the baseband signals sampled by the A/D converters 20 involves a 2-stage down-conversion process. Both of these downconversion stages are subject to frequency errors and feedback loop delays. In order to obtain an acceptable Signal to Noise Ratio (SNR) in the raw sample streams IX, QX output from the A/D converters 20, the Rx LO must be controlled to maintain a desired frequency offset Δf between the Rx LO light and the Tx optical carrier frequency f0, and the electronic LO must be controlled to maintain an output frequency f2 that closely matches the frequency offset Δf This cannot be done in the presence of moderate to severe optical channel impairments, because these impairments must be compensated before a reasonably accurate estimate of the optical carrier frequency f0, and thus the frequency offset Δf, can be obtained. However, this estimate of the frequency offset Δf is needed to enable the A/D converters 20 to produce raw sample streams IX, QX with a high enough SNR that the impairments can be compensated. Accordingly, receivers implementing optical heterodyne down-conversion have not typically been implemented outside of laboratory conditions.
Techniques for carrier recovery that overcome limitations of the prior art remain highly desirable.